Governments already have everyone's ID, including DOB. They say that the problem is non-adults accessing adult sites and services. So therefore, the sites need to know that users are over 18 (or the selected government age). There should be a standardized government ID service/API that allows a person to let it disclose their age (or other user selected information) to a requesting site/service. That's all that is ne…
The german gov id supports that. They have a PKI and the id is a smart card with a cert and private key on it [0]. It lets you answer the question "are you over 18" with a zero knowledge proof. I guess it only proves you have in your possession a valid id AND know the PIN to it, but that should be fine. France apparently has this, too, according to the article. [0] https://www.personalausweisportal.de/Webs/PA/EN/gove…
My German isn't that great, but I can't find any sources that state that the German eID can use ZKPs directly. From what I can tell, it uses basic signatures, and the new/upcoming wallet app will take care of ZKP generation. But maybe I'm missing a source in German?